1. Technical Field
The present invention relates to a sample milling/observing apparatus including an ion gun which irradiates an ion beam on a sample to form a cross-section to be observed (hereinafter, to be referred to as observed section) and a method of observing a sample using the milling/observing apparatus for the sample.
2. Related Art
As an electronic microscope which observes a sample, a scanning electronic microscope (SEM) is known. When a sample is observed by the electronic microscope of this type, in general, the sample is placed on a predetermined stage, and an electron beam is focused on an observed surface of the sample in accordance with a tilt state of the observed surface. The electron beam is focused by changing a magnetic field of an object lens.
In this case, the stage is arranged to be able to be rotated in two axial directions. There is proposed an electronic microscope in which even though the tilt state of the observed surface of the sample changes with respect to the two axes, focusing of an electron beam is performed in accordance with the change of the tilt state (for example, see Japanese Laid-open patent publication NO. 2000-100367). In this electronic microscope, tilt angles of the axes are detected by an encoder, the magnetic field of the object lens is changed depending on the tilt angles to correct a focal point.
There is also proposed an electronic microscope in which currents of an object lens are changed in accordance with tilt states of observed surfaces of a sample having two observed surfaces (for example, see Japanese Laid-open patent publication NO. 2001-210263). In the electronic microscope, changed positions of tilts of the observed surfaces are calculated to change the current of the object lens such that the two observed surfaces are displayed by one image.
There is an observing method which performs a cutting operation to an observed section by focused ion beam milling (FIB) a plurality of times to continuously update the observed section. This observing method is used to make it possible to observe a change of the section in a cut direction of the sample.
However, even in the electronic microscope of any one of Japanese Laid-open patent publication NOS. 2000-100367 and 2001-210263, when an observed section is cut by an ion beam irradiated from an ion gun, an electron beam goes out of focus by an amount of cut. More specifically, in each of the electronic microscopes, focal correction is performed in accordance with a change of the sample in an tilt direction. For this reason, the electronic microscopes cannot cope with a case in which the observed section of the sample is cut to change the depth thereof. In this manner, each time the observed section is updated by the ion beam, the electron beam must be focused and make observation of the change of the section cumbersome.